1. Field of the Invention
The present invention relates to a data backup system and a data duplicating apparatus employed in the backup system used in a variety of data processing systems. More particularly, the present invention relates to a data backup system and a data duplicating apparatus employed in a data backup system that can be used for making a backup of data without sacrificing availability of the data processing system adopting the backup system.
2. Related Art
Traditionally, in order to raise the reliability of data in a variety of data processing systems, there is adopted a double-storage configuration in which the data is stored in a storage device of the system and a duplicate of the data is stored in a backup device. The backup device monitors commands issued by a host apparatus to the storage device. When a transfer of data from the host apparatus to the storage device is detected, data being stored into the storage device is also taken in by the backup device. In this way, a time it takes to make a backup of data being stored is concealed in a time it takes to store the data into the storage device. As a result, the availability of the data processing system is improved. In addition, even though the access speed of the backup device is not so high as that of the storage device in many cases, by providing the backup storage with a high-speed buffer area such as a cache memory in such a case, the difference in access speed between the backup device and the storage device can be absorbed so that it is possible to make operations of the backup device keep up with transfers of data in the data processing system.
Take a RAID system, which has been becoming popular, as an example. The RAID system is a system provided with a plurality of low-cost disc drives and intended for further improving a high-speed response characteristic and redundancy or reliability of data.
In particular, a RAID1 system is a system for improving redundancy or reliability of data. The RAID1 system adopts the so-called disc mirroring technique whereby data is stored in a pair of disc drives. Specifically, the data is stored in a data storage disc drive while a duplicate of the data is stored in a backup disc drive. In a data write operation, data is stored in both the disc drives at the same time. Thus, the host apparatus carries out an operation to write data into the pair of disc drives in the same way as the ordinary operation to store data without being aware of the fact that the backup disc drive is used as a redundant disc drive. In a data read operation, on the other hand, an access can be made to any individually selected disc drive. Thus, if one of the disc drive in the pair is out of order, the access can be switched to the other disc drive, thereby a normal data read operation can be carried out in a system implementing data redundancy or data reliability.
However, the host apparatus does not receive any response from the backup disc drive except a report such as a result of a self diagnosis carried out by each of the disc drives at a system build-up time in spite of the fact that, when the host apparatus writes data into the data storage disc drive controlled by the host apparatus, the data is also written into the backup disc drive at the same time without the need for the host apparatus to be aware of the fact that one of disc drives in the pair is a duplicate of the other. In order to verify that data has been correctly written into the disc drive recognized as the storage device, processing such as detection or correction of an error is carried out in the course of a normal data write operation. On the other hand, there is no verification unit for verifying that data has been correctly stored at the same time into the backup disc drive during the data write operation. There is raised a problem of inability to assure reliability of data written into the backup disc drive.
For the reason described above, in order to assure reliability of data written into the backup disc drive, it is necessary for the host apparatus to execute a procedure to read out data back from the backup disc drive. Even though the availability of the data processing system is improved by virtue of writing of data into both the disc drives at the same time in a data write operation, in order to assure reliability of data written into the backup disc drive, it is necessary to read out the data back after writing it in order to verify that the data has been written correctly. There is raised a problem of worsened availability of the data processing system.
In addition, it is generally flows of data transfers between the host apparatus and the data storage disc drive that are controlled. That is, such control of flows is not applied to the backup disc drive, which has an access speed lower than the data storage disc drive. By setting a configuration to provide the backup storage with a high-speed buffer area such as a cache memory, however, the difference in access speed between the backup device and the storage device can be absorbed. Nevertheless, since the buffering performance is determined by the size of the buffer area, in an operation to write data with an amount exceeding the size of the buffer area, the buffer area cannot accommodate the data, raising a problem of a data overflow or a problem of a data underflow in the case of a read operation. In order to solve the problems such as a data overflow and a data underflow, it is necessary to allocate a buffer area of a large size. However, such a buffer area causes another problem of an increased number of components composing the backup disc drive and a large number of such components gives rise to another problem of a rising cost.